American Society of Anesthesiologists, 2006 Annual Meeting
A851
October 16, 2006
9:00 AM - 11:00 AM
Room Hall E, Area C

Hypercapnic Hyperventilation Shortens Recovery Time after Isoflurane and Sevoflurane Anesthesia
Nishant A. Gopalakrishnan, Ph.D., Derek J. Sakata, M.D., Joseph A. Orr, Ph.D., Dwayne R. Westenskow, Ph.D.

Introduction: Hypercapnic hyperventilation can be used to rapidly remove anesthetic agent from the lungs, increase clearance of anesthetic from the brain and decrease emergence time from inhaled anesthetic agents. The objective of this study is to measure BIS as an indicator of anesthetic depth after isoflurane or sevoflurane anesthesia when hypercapnia or hypocapnia were maintained with hyperventilation during emergence.

Methods: We evaluated BIS rise times with hypocapnic and hypercapnic hyperventilation during recovery from anesthesia. Prior to emergence, 7 pigs were anesthetized with 2 MACPIG of Isoflurane(3.1%) and 4 pigs were anesthetized with 2 MACPIG of sevoflurane (3.94%) for two hours. Three emergences, 2 hypercapnic(55 mmHg) and one hypocapnic (22 mmHg) were performed for each animal and were separated by 30 minutes of anesthesia at 2MACPIG. All animals were hyperventilated during emergence by doubling respiratory rate from 10 to 20 breaths per minute and increasing fresh gas flow to 10 L/min. Hypercapnia was maintained by injecting CO₂ into the inspired gas under feedback control or by adding dead space to the airway (with activated charcoal to remove anesthetic from the rebreathed gas). The order of emergence was randomized to minimize the effects of a prior emergence. BIS recorded from the time the vaporizer was turned off was normalized using the pre-emergence BIS (average BIS 2 minutes prior to turning off the vaporizer) and maximum BIS observed during emergence.

Normalized BIS =(BIS -pre-emergence BIS)/(maximum BIS -pre-emergence BIS)

BIS data was normalized within each animal to remove the inter-animal variability of the BIS signal. Normalization assumes that the pre-emergence BIS and maximum BIS during emergence did not differ significantly within each animal. The time for the normalized BIS to rise to 0.95 was compared to the time to movement of multiple limbs. The order of emergence was randomized to minimize effects of a prior emergence.

Results: With isoflurane, the average time for the normalized BIS to rise to 0.95 was 5.38 minutes with hypercapnia and 14.47 minutes with hypocapnia. With sevoflurane, the average time for the normalized BIS to rise to 0.95 was 6.22 minutes with hypercapnia and 13.47 minutes with hypocapnia. With isoflurane, the average time to limb movement was 6.1 minutes with hypercapnia, 19.1 minutes with hypocapnia and for sevoflurane, 6.7 minutes with hypercapnia and 7.1 minutes with hypocapnia. BIS rise times were not statistically different from the time to movement of multiple limbs(p<0.05). The standard deviation of the time for the normalized BIS to rise to 0.95 fell from 5.5 to 0.88 minutes for isoflurane and from 2.67 to 1.03 minutes for sevoflurane when hypercapnia was maintained during emergence.

Discussion: BIS rose faster and more predictably when hypercapnia was maintained during hyperventilation. Hypercapnia increases the rate of fall in cerebral agent concentration, shortening the transition time from asleep to awake state and thus makes emergence time more predictable.